[C#] Deduplicate OnDisposeCall and improve implementation (#626)

* Initial plan

* Extract ActionDisposable into its own file and use it from both Client.cs and Session.cs

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

dotnet/src/ActionDisposable.cs

@@ -0,0 +1,24 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------------------------------------------*/
+
+namespace GitHub.Copilot.SDK;
+
+/// <summary>
+/// A disposable that invokes an action when disposed.
+/// </summary>
+internal sealed class ActionDisposable : IDisposable
+{
+    private Action? _action;
+
+    public ActionDisposable(Action action)
+    {
+        _action = action;
+    }
+
+    public void Dispose()
+    {
+        var action = Interlocked.Exchange(ref _action, null);
+        action?.Invoke();
+    }
+}

dotnet/src/Client.cs

@@ -1561,22 +1561,3 @@ public class ToolResultAIContent(ToolResultObject toolResult) : AIContent
 {
     public ToolResultObject Result => toolResult;
 }
-
-/// <summary>
-/// A disposable that invokes an action when disposed.
-/// </summary>
-internal sealed class ActionDisposable : IDisposable
-{
-    private Action? _action;
-
-    public ActionDisposable(Action action)
-    {
-        _action = action;
-    }
-
-    public void Dispose()
-    {
-        var action = Interlocked.Exchange(ref _action, null);
-        action?.Invoke();
-    }
-}

dotnet/src/Session.cs

@@ -246,7 +246,7 @@ void Handler(SessionEvent evt)
     public IDisposable On(SessionEventHandler handler)
     {
         _eventHandlers.Add(handler);
-        return new OnDisposeCall(() => _eventHandlers.Remove(handler));
+        return new ActionDisposable(() => _eventHandlers.Remove(handler));
     }
 
     /// <summary>
@@ -600,11 +600,6 @@ await InvokeRpcAsync<object>(
         }
     }
 
-    private class OnDisposeCall(Action callback) : IDisposable
-    {
-        public void Dispose() => callback();
-    }
-
     internal record SendMessageRequest
     {
         public string SessionId { get; init; } = string.Empty;